I have created a boot 'disk' for
Computer A. If I were to then take A's SSDs and stick them in a better Computer B (replacing B's SSDs) then use the boot disk to fire up B...would that essentially replicate Computer A on the newer shinier box?

If the SSD fits, and, B can see it, then it may boot properly, the issues can be drivers, if the peripherals in Computer B are of different design/manuf, then you may have to spend time updating drivers to get it to see all the ports, modem, cd drive, network card etc.

Yeah, that can work. If the boot disk has a full OS and isn’t locked to specific hardware, it should fire up on the new machine just fine. Your SSDs from Computer A will carry over your files and apps, so it’ll feel like the same system.

Only thing is, Computer B might need to install new drivers the first time it boots, especially if the hardware’s a lot different. And if it’s Windows, it might ask you to reactivate because of the motherboard swap

What OS are you talking about? If you have the SSD drives configured in a RAID, you may have to do something "special" before you move them, for example, with several varieties of Unix RAID software (e.g. ZFS and its cousins), you may have to "export" the arrays before you can "import" them on the new machine. This isn't always necessary, but you can't know until it is. It is because device and controller designations often differ among hardware.

If you hope to boot from the SSDs, there are other considerations, but the details depend a lot on the OS you want to run.

It's Windows 10. Peripherals are all Corsair and LG, so should be no issue there.

Any background on why you want to do this? I have stuck older drives in different computers and had them boot, so its possible. HRK pointed out some issues you will likely encounter, not to mention the OS license on computer A is tied to that computers hardware. Assuming computer B has the same OS license it might activate, but I don't know. If the OS doesn't activate I don't think you will get OS updates and will be limited on some available options in the OS.

Basically I have a second PC I used (barely) for DCS. It has a better CPU, graphics card, etc. but I rarely use it. DCS is a bear to learn and even now that I'm retired, I don't devote much time to it. It has also run a little wonky, I THINK due to the version of Windows on it.

Anyway, while cleaning them, I got the bright idea of swapping the SSDs (M2s) so I could take advantage of the better hardware. Like on the A computer, I average around 55 fps playing ArmA REFORGER. On the B, average is like 98.

Digital Combat Simulator?

Does computer B have an OS installed?

Why not just install DCS on computer B?

B is the DCS. Runs Windows...not 100% sure if 10 or 11.

Computer A is my "normal use" PC. Don't know the specs off the top of my head, but the graphics card is a Radeon 6900XT.

Computer B is the DCS computer. Has a 7900XTX (and a better CPU).

As I'm not playing DCS much at all, I'm wanting basically to swap the SSDs between the two, turning B into A.

Swapping SSD's is likely going to cause you more pain than it's worth. Without knowing more, it seems the best option would be to reinstall whatever apps you need on Computer B, and transfer the data over.